I have a T-34/85 converted to HL S&S&airsoft, and run it using the plastic tracks, and metal gears.

I have changed the stock motors with Graupner 300 6V motors after burning one stock motor (don't know how, I was running inside at home). After that the tank became fast at top speed (faster than all my other HL tanks).

I had some time ago a problem on the 2nd metal gear, where 1/3 of the theet did break all of a sudden. Don't know why.
Now I have rebuilt it using a mix of nylon (the first gear after the motor) and metal gears, and I will test it for some time.

If you keep the gearbox well lubricated , it will last enough, and after that you can always rework the box and put bushes inside the worn plastic.

I suggest you to go with plastic tracks, though. There's no need for metal ones.

One part that you should worry about is the idler: due to its shape (the working tension adjuster) it will bend and possibily break, especially if you hit ground obstacles with front wheels. I have replaced both the idler and tensioner with metal parts, and I am happy with it.